In direct-broadcast-from-satellite (DBS) systems, a frequency-modulated information signal, which is generally a video signal, is transmitted on a carrier at super high frequencies (SHF) from a satellite to a large number of DBS receivers on the ground. In order to avoid interference with signals being transmitted by ground-based systems, the video signal may be composed with an energy-dispersal signal before frequency modulation. The energy-dispersal signal has a triangular waveform synchronized with the video signal, for example at the frame rate, and the resultant composed signal, when frequency modulated and transmitted on a carrier, may be effectively transmitted without disturbing ground-based transmission systems.
The energy-dispersal signal must then be rejected from the received signal at the DBS receiver. It has been found that in order to obtain a good video picture, the energy-dispersal signal rejection ratio must be at least 50 dB. The result of partial rejection (low rejection ratio) of the energy-dispersal signal is a sag at the vertical rate in the video signal. One way to reject the energy-dispersal signal is to supply the demodulated signal to a peak-clamp circuit or a pulse clamp circuit, such as are used to fix the peak level of the video synchronizing signals. In order to achieve the desired rejection ratio of at least 50 dB, the time constant of the clamp circuit must be short, since the use of a clamp circuit with a relatively long time constant results in only partial rejection of the energy-dispersal signal.
In practical terms, peak clamp circuits with short time constants are difficult to realize. As an alternative, pulse clamp circuits with short time constants are more easily realizable, and therefore the desirably large rejection ratio is achievable thereby. However, pulse clamp circuits with short time constants are subject to misclamping caused by noise generated in the frequency demodulator, particularly when the carrier-to-noise ratio (C/N) is bad. The result is that horizontal line-shaped noise appears in the picture and the quality of the picture is severely degraded. Therefore, these prior art methods for rejecting the energy-dispersal signal are ineffective to provide both a high rejection ratio and good picture quality.
It has been proposed, by persons having an obligation to assign to the assignee of the present application, to provide a clamp circuit whose time constant is selectively made longer when the C/N ratio is low. This proposal is described in Japanese Patent Application No. 61-201139, assigned to the present assignee. By using this technique, misclamping is avoided, but a high rejection ratio is still unobtainable when the C/N ratio is low.